A variety of touch screen devices including personal digital assistants (PDA), tablet computers, mobile phones, kiosks, handheld gaming consoles and the like have become popular. A touch screen device may comprise a pressure sensitive transparent panel overlaying a display such as a liquid crystal display (LCD) screen. The pressure sensitive transparent panel is capable of detecting the pressure from a finger touch and forwarding the detected signal to a processor coupled to the panel. The processor may process the detected signal and determine whether there is a finger touch and the location of the finger touch.
The prevailing touch screen technologies may be divided into two categories, namely resistive touch screen techniques and capacitive touch screen techniques. A touch screen system based upon a resistive touch screen technique may comprise two conductive layers, which are separated from each other by an air gap when there is no touch present on the surface of the resistive touch screen. When a finger touches the screen, the pressure may cause two conductive layers to contact each other so that the resistance between the two conductive layers is lowered down as a result. A sensing circuit may detect the resistance variation and determine the location of the finger touch accordingly.
A capacitive touch screen may comprise a plurality of transmit lines TX and receive lines RX arranged in rows and columns. The transmit lines may be coupled to a signal source and arranged in parallel in a first direction. The receive lines may be coupled to a sensing circuit and arranged in parallel in a second direction. The first direction is orthogonal to the second direction. Therefore, the transmit lines may intersect with the receive lines to form a plurality of capacitors. As a finger touches a point of the capacitive touch screen, the capacitance at the point may vary as a result. Such capacitance variation at that point causes a change of the detected signal at the sensing circuit coupled to the receive lines. As a result, the sensing circuit may determine whether there is a finger touch. Furthermore, if a finger touch exists, the sensing circuit may detect the location of the finger touch.
The touch screen devices may be powered by a battery charger when a rechargeable battery of the touch screen device is depleted. However, the battery charger may cause noise interference. In particular, the battery charger is coupled to a utility power line. The noise from the utility power line may penetrate into the touch screen system and contaminate the transmit lines TX through the battery charger. For example, when a battery charger is connected to a touch screen system, intermittent bursts of noise may occur at electrodes of the touch screen. This kind of intermittent bursts of noise is commonly known as electrical fast transient (EFT) noise, which may trigger false finger touches. In order to prevent EFT noise from falsely triggering a finger touch event, a touch screen system may take a variety of EFT noise tests before the design of the touch screen system is finalized. The EFT noise test should be conducted under conditions specified by IEC 61000-4-4.